Diaminonaphthalene functionalized LUS-1 as a fluorescence probe for simultaneous detection of Hg2+ and Fe3+ in Vetiver grass and Spinach

One of the important problems in the environment is heavy metal pollution, and fluorescence is one of the best methods for their detection due to its sensitivity, selectivity, and relatively rapid and easy operation. In this study, 1,8-diaminonaphthalene functionalized super-stable mesoporous silica (DAN-LUS-1) was synthesized and used as a fluorescence probe to identify Hg2+ and Fe3+ in food samples. The TGA and FT-IR spectra illustrated that 1,8-diaminonaphthalene was grafted into LUS-1. XRD patterns verified that the LUS-1 and functionalized mesoporous silica have a hexagonal symmetrical array of nano-channels. SEM images showed that the rod-like morphology of LUS-1 was preserved in DAN-LUS-1. Also, surface area and pore diameter decreased from 824 m2 g⁻1 and 3.61 nm for the pure LUS-1 to 748 m2 g⁻1 and 3.43 nm for the DAN-LUS-1, as determined by N₂ adsorption–desorption isotherms. This reduction demonstrated that 1,8-diaminonaphthalene immobilized into the pore of LUS-1. The DAN-LUS-1 fluorescence properties as a chemical sensor were studied with a 340/407 nm excitation/emission wavelength that was quenched by Hg2+ and Fe3+ ions. Hg2+ and Fe3+ were quantified using the fluorescence response in the working range 8.25–13.79 × 10–6 and 3.84–10.71 × 10–6 mol/L, with detection limits of 8.5 × 10–8 M and 1.3 × 10–7 M, respectively. Hg2+ and Fe3+ were measured in vetiver grass and spinach. Since the Fe3+ quenching can move in the opposite direction with sodium hexametaphosphate (SHMP) as a hiding compound for Fe3+, consequently, the circuit logic system was established with Fe3+, Hg2+, and SHMP as inputs and the fluorescent quench as the output.

The morphology of LUS-1 and DAN-LUS-1 was investigated by SEM micrographs.Figure 1a shows SEM micrograph of the unmodified LUS-1 and Fig. 1b shows SEM micrograph of the DAN-LUS-1.SEM images showed that the LUS-1 has a rod-like morphology which is preserved in DAN-LUS-1 46 .Also, in Fig. 1b, it is clear that the 1,8-diaminonaphtalene is dispersed on the surface of channels.
XRD pattern of LUS-Pr-Cl, LUS-1, and DAN-LUS-1 displayed the same ordered, (200), (110), and (100) reflections, which displayed a hexagonal symmetrical array of nano-channels (Fig. 2).It is suggested that during the surface modification procedure, the LUS-1's hexagonal structure is preserved, with no pore wall collapse.The peak strength (100) was reduced with a marginal peak shift after immobilizing the functionalized LUS-1 materials.The reason is the pore-wall scattering contrast difference and the organic groups irregular coverage on nanochannels 47,48 .
Figure 3 gives the FT-IR spectra of LUS-Pr-Cl, LUS-1, and DAN-LUS-1.Mesoporous show peaks at 960, 800, 1100, and 3000-3450 cm −1 which belong to symmetric stretching of Si-O, asymmetric stretching vibrations of Si-O-Si, symmetric stretching of Si-OH, and stretching vibrations of OH, respectively 48 .Peak at 1650 cm −1 related to the physisorbed H 2 O onto the LUS-1.In LUS-Pr-Cl and DAN-LUS-1 spectra, peaks at 2880-2990 cm −1 are seen which are associated with the stretching vibrations of -CH 2 -of propyls 49 .Also, new peaks at around 3550 cm −1 , 1580 cm −1 and 1700 cm −1 which are seen in DAN-LUS-1 spectrum, are assigned to the NH stretching vibration, NH bending 50,51 and C=C aromatic stretching vibrations 52 .These new peaks confirm the successful attachment process.
Figure 4 displays the N 2 adsorption-desorption isotherms of LUS-Pr-Cl, LUS-1, and DAN-LUS-1 compounds.All the materials display "type IV" isotherm with an "H1-kind" hysteresis representative of the cylindrical construction which is related to mesoporous compounds matching IUPAC adsorption-desorption isotherms 53 .
Table 1 offers the samples' textural parameters counting pore diameter (BJH), specific surface area (BET), and total pore volume.The decrease in parameters indicates the effective attachment of organic parts on the inner surface.
The thermogravimetry analysis was done to evaluate the organic content in LUS-Pr-Cl, LUS-1, and DAN-LUS-1.Figure 5 offers TGA curves of all of them.The early increase in the LUS-1 and LUS-Pr-Cl weight from 100% was associated with the buoyancy effect 46 .The first weight loss was less than 150 °C owing to the evaporation of water on the surface 48,54 .Thus, the considerable weight losses between 200 and 800 °C associated with the decomposition of the organic species 52 .In this range, the weight loss for LUS-Pr-Cl, and DAN-LUS-1 was related to the attached chloropropyl trimethoxysilane (13%), and the attached 1,8-diaminonaphthalene (25%), respectively.

Fluorescence studies
DAN-LUS-1 was applied as a fluorescence sensor to determination of Hg 2+ and Fe 3+ ions in water samples.The emission spectra of a dilute suspension of DAN-LUS-1 in water (0.05 g L −1 ) were investigated at the excitation  wavelength (λex) 340 nm.There was an emission peak at 407 nm (Fig. 6).Adding various metal ions to DAN-LUS-1 suspension changed the emission intensity.A significant reduction in the amount of the emission signal at 407 nm was observed only after addition of Hg 2+ and Fe 3+ ions and then changed partially by adding Al 3+ , Ca 2+ , Co 2+ , Cd 2+ , Ag + , Pb 2+ , Ba 2+ , Ni 2+ , Mn 2+ , Mg 2+ , Zn 2+ , Cu 2+ , Na + and Fe 2+ ions (Fig. 6).These findings indicate that DAN-LUS-1 can be used as a fluorescence sensor for detecting and determining Hg 2+ and Fe 3+ ions in aqueous media.Quenching effect of metal ions can be attributed to the paramagnetic nature of these ions or the heavy metal ion effect 55 .
The fluorescence behavior of the suspension of DAN-LUS-1 in absence and presence of Hg 2+ over the pH range 2-9 and in presence of Fe 3+ over the pH range 2-5 was studied.The pH of the solutions was adjusted by adding HCl or NaOH.As can be seen in Fig. 8, with addition of Fe 3+ and Hg 2+ , a significant fluorescence quenching was occurred.Fluorescence intensity of DAN-LUS-1 (at 407 nm) increases with increasing pH.The quenching of Hg 2+ increases with the increase of pH from 2 to 5 and remains almost constant after that.The quenching Fe 3+ is almost constant in the range of pH 2 to 5. Therefore, in the simultaneous measurement of Hg 2+ and Fe 3+ , pH = 5 is the best pH.Of course, if the goal is only to measure Hg 2+ , a higher pH where the fluorescence intensity of the DAN-LUS-1 is higher can be used to increase the sensitivity of the measurement.
The recognition capacity of DAN-LUS-1 toward Hg 2+ and Fe 3+ ions was determined in the aqueous solution of the DAN-LUS-1 suspension via titration test.When the Hg 2+ and Fe 3+ ions concentration increases (Figs.9a  and 10a), the fluorescence intensity reduces.The Stern-Volmer plots were revealed in Figs.9b and 10b for Hg 2+ and Fe 3+ , respectively.A linear relation between fluorescence intensity and Hg 2+ and Fe 3+ ions concentration was obtained by the equations y = 0.3511x + 0.7802 and y = 0.2166x + 0.4897 with a regression coefficients, R 2 = 0.9909 and R 2 = 0.9887, respectinely.The linear response spans a concentration of 8.25-13.79× 10 -6 , 3.84-10.71× 10 -6 mol/L of Hg 2+ and Fe 3+ , respectinely.Moreover, the detection limit was based on the DL = 3S d /m equation, in Figure 6.Fluorescence emission spectra of the suspension of DAN-LUS-1 in water (0.05 g L −1 ) after adding Hg 2+ , Fe 3+ , Al 3+ , Ca 2+ , Co 3+ , cd 2+ , Ag + , Pb 2+ , Ba 2+ , Ni + , Mn 2+ , Mg 2+ , Zn 2+ , Cu 2+ , Na + and Fe 2+ ions (λ ex = 340 nm, λ em = 407 nm).www.nature.com/scientificreports/which S d shows the standard deviation of blank and m represents the slope of calibration curve (Stern-Volmer plot).The detection limits were 8.5 × 10 -8 M and 1.3 × 10 -7 M for Hg 2+ and Fe 3+ determination, respectively.Recyclability is a highly important criterion for assessing the fluorescence sensor's performance in practical applications.The utilized DAN-LUS-1 are easily extracted from the solution using a fast centrifugalizing, repeatedly cleaned in distilled water, and then dried at 60 °C.Recycled DAN-LUS-1 was re-dispersed in the reaction media for subsequent experiments.After three consecutive cycles, the fluorescence of reused DAN-LUS-1 showed no detectable change.This indicates the stability and reusability of the proposed DAN-LUS-1 sensor as a fluorescence sensor towards Hg 2+ and Fe 3+ .

Mechanism of quenching
The higher sensitivity of DAN-LUS-1 toward Fe 3+ and Hg 2+ ions is possibly caused by their fast chelating kinetics and higher affinity with nitrogen atoms of the probe.Unmodified LUS-1 is non-fluorescent, but 1,8-diaminonaphthalene is a fluorescent molecule.When LUS-1 is functionalized with 1,8-diaminonaphthalene, DAN-LUS-1 will also be fluorescent.Therefore, the emission bands are from 1,8-diaminonaphthalene molecules.The quenching of the fluorescence emission can be attributed to the complex formation between metal ions and DAN-LUS-1 as a ligand molecules.DAN-LUS-1 most likely binds with Hg 2+ and Fe 3+ via the nitrogen atoms of functional group of 1,8-diaminonaphthalene in mesoporous framework which potentially leading to quenching effect due to the photoinduced electron transfer (PET) mechanism 56 .Electron transfer occurs from the excited DAN-LUS-1 (as a Lewis base) to the electron-deficient metal ions (as a Lewis acid).Also, the observed quench created by Fe 3+ is possible their paramagnetic nature.Forbidden intersystem crossings are simplified by paramagnetic species, leading to quenching the fluorescence emission.The heavy atom effect can be considered to explain fluorescence quenching by Hg 2+57 .Figure 11 indicates the proposed fluorescence sensing mechanism of DAN-LUS-1 by Hg 2+ and Fe 3+ .

Logic behaviour of DAN-LUS-1
A binary logic circuit can be made from the correspondence between Hg 2+ , Fe 3+ , and sodium hexametaphosphate (SHMP) as the inputs and the fluorescent quench of DAN-LUS-1 at 407 nm as the output (Fig. 12).SHMP is a masking agent to block Fe 3+ .The circuit was deliberate according to the standard expression of products-of-sum.The Products-of-sum generate the truth table, where the presence and absence of the inputs were illustrated by 0 and 1, respectively.Concerning the output, 0 displays a fluorescence emission, while 1 clarifying an emission quenching.Consequently, five out of eight probable combinations led to a fluorescence quenching.
Hiding effect of SHMS for Fe 3+ was investigated with adding Fe 3+ and SHMS to DAN-LUS-1.The probe can be switched from "on" to "off " by adding SHMP to a solution containing DAN-LUS-1 and Fe 3+ .As can be seen in Fig. 13, the Fe 3+ quenching can move in the opposite direction with adding SHMP as a hiding compound.But, the Hg 2+ quenching does not change with adding SHMP.These results enable the simultaneous detection of Fe 3+ and Hg 2+ using a chemosensor and a logic circuit.

Determination Hg 2+ and Fe 3+ in Vetiver Grass and Spinach
The proposed fluorescence sensor and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) were applied to determine Fe 3+ and Hg 2+ in vetiver grass and spinach.The results obtained are presented in Table 2. Good agreement between the two methods indicate the capability of the proposed methode for the determination of Fe 3+ and Hg 2+ in the food samples and approve the accurateness of the suggested technique and its independency from matrix effects.www.nature.com/scientificreports/

Comparison section
A comparison was made between the several Fe 3+ and Hg 2+ fluorescence sensors using various sensing materials and the proposed sensor (DAN-LUS-1) developed in this study (Table 3) [58][59][60][61][62] .Overall, the developed fluorescence sensor is superior in detection limit to the mentioned sensors.Also, the proposed sensor is one of the few sensors that allows the simultaneous detectionof Fe 3+ and Hg 2+ .

Reagents and apparatus
All materials and tools are describes in Sect.S1.
Figure12.The schematic of the truth table and logic circuit for DAN-LUS-1 with Hg 2+ , Fe 3+ , and SHMP as the inputs.www.nature.com/scientificreports/

Hg 2+ and Fe 3+ determination in real samples
Vetiver grass and spinach were utilized as real samples to use the fluorescence probe and determine Hg 2+ and Fe 3+ .In this process, for 3 months period, all vetiver grass was watered with Hg 2+ (30 mg/L) solution.After this period, they were cutted and washed.Then they were dried at 70 ℃ in an oven.Dried vetiver (5g) was digested with 15 mL hydrochloric acid (HCl) and 5 mL nitric acid (HNO 3 ), and 10 mL hydrogen peroxide (H 2 O 2 ).The sample was heated on hot plate within the fume chamber during the digestion procedure, for 2 h and each hour 10 mL of the solution were added owing to drying the sample.H 2 O 2 was added little by little during the digestion process.After finishing the digestion process, samples were cooled down and pH adjusted by NaOH.The ultimate volume of the solution was set to 25 mL with distilled water.Spinach was dried and digested according to vetiver digestion method.All procedures and experiments related to the vetiver grass and spinach are complied with relevant institutional, national, and international guidelines and legislation.

Conclusion
In this work, a novel fluorescent sensor was prepared in terms of the surface modification of LUS-1 by a 1,8-diaminonaphthalene (DAN-LUS-1) for high sensitive detection of Hg 2+ and Fe 3+ ions.The DAN-LUS-1 was synthesized and characterized.FT-IR and TGA spectra represented the presence of 1,8-diaminonaphthalene groups in the silica system.The XRD pattern displayed a hexagonal symmetrical array of nano-channels suggested that DAN-LUS-1 was characteristic of mesoporous silica.The SEM images of mesoporous materials represented the rope-shaped morphology.Consequently, Hg 2+ and Fe 3+ were quantified using the fluorescence response at 407 nm in working range 8.25-13.79× 10 -6 and 3.84-10.71× 10 -6 mol/L, respectively.Detection limits were 8.5 × 10 -8 and 1.3 × 10 -7 mol L −1 for Hg 2+ and Fe 3+ determination, respectively.The introduced sensor was successfully applied to measure the concentration of Hg 2+ and Fe 3+ ions in the Vetiver Grass and Spinach.Satisfactory results were obtained, which were consistent well with the results from the standard ICP-MS technique.
Under the individual actions of the of Hg 2+ and Fe 3+ , and their combination, a solid support molecular logic circuit was obtained successfully, producing one optical outputs while stimulating three chemical inputs.In this arrangement, a combinatorial system is developed simultaneously acting as a logic circuit and as a chemosensor in terms of grafted molecules on the solid support.
The results revealed a fast and easy method to determine Hg 2+ and Fe 3+ at trace levels and construct a combinatorial logic circuit.

Figure 11 .
Figure 11.Schematic of the DAN-LUS-1 quenching mechanism in the presence of Hg 2+ and Fe 3+ .
https://doi.org/10.1038/s41598-024-66453-8www.nature.com/scientificreports/mL) and different amounts of the solution of ion.The fluorescence data of the samples were attained within the wavelength range of 250-600 nm and excitation wavelength of 340 nm.

Table 2 .
The detected concentrations of Hg 2+ and Fe 3+ in real specimens by proposed sensor and ICP-MS.

Table 3 .
Comparison with the other Fe 3+ and Hg 2+ fluorescence sensors.